High pressure presses have been used for decades in the manufacture of, for example, synthetic diamond. Such presses are capable of exerting a high pressure and high temperature on a volume of carbonaceous material to reproduce the conditions that create natural diamond inside the earth. Known designs for high pressure presses include, but are not limited to, the belt press, the tetrahedral press, and the cubic press.
FIG. 1 shows a basic design for a conventional cubic press 10 known in the art. The press 10 generally includes six press bases 12, with each press base 12 aligned relative to a common central region 14. Each press base 12 includes a piston cavity 13 for receiving a piston 16, the cavities 13 being open towards the common central region 14. Cubic press 10 also includes tie bars 18 extending between each pair of adjacent press bases 12.
During operation of cubic press 10, a piston 16 is thrust out of a piston cavity 13 in each press base 12 towards common central region 14. As the pistons 16 move forward towards common central region 14, pistons 16 apply pressure to each side of a cubic volume of carbonaceous material located at common central region 14. The pressure exerted on the cubic volume of carbonaceous material at common central region 14 tends to result in counter forces acting on the various components of cubic press. Left unchecked, these counter forces can lead to component malfunction and failure such as, for example, the creation and propagation of cracks in press bases 12. Tie bars 18 are included in cubic press 10 in an attempt to stabilize cubic press 10 in the face of these counter forces.
FIGS. 2 and 3 illustrate the manner in which tie bars 18 of a known cubic press are positioned between adjacent press bases 12 (12a, 12b). With respect to FIG. 2, press bases 12 generally include a plurality of tie bar receiving holes 17. Tie bar receiving holes 17 extend from an outer surface 19 of press base 12 to a tie bar cavity 21 of press base 12. The size of tie bar receiving holes 17 is approximately equal to the size of tie bars 18 so that tie bars 18 fit snugly in tie bar receiving holes 17 when they are passed through tie bar receiving holes 17.
As shown in FIG. 3, first and second press bases 12a and 12b are positioned so that tie bar receiving hole 17a in first press base 12a is aligned with tie bar receiving hole 17b in second press base 12b. With first and second press bases 12a and 12b being aligned, tie bar 18 is passed through tie bar receiving hole 17a in first press base 12a and tie bar receiving hole 17b in second press base 12b. Tie bar 18 has a first end 18b and a second end 18a opposite first end 18b. Second end 18a has an enlarged base that is larger than the opening of tie bar receiving hole 17a, so that when tie bar 18 is positioned through both tie bar receiving holes 17a and 17b, the enlarged base of second end 18a at least partially abuts against first press base 12a. First end 18b is threaded, so that when it protrudes out of tie bar receiving hole 17b of second press base 12b, a nut 22 having threads compatible with those of first end 18b can be assembled on to first end 18b of tie bar 18.
Nut 22 may be assembled on to first end 18b of tie bar 18 until nut 22 at least partially abuts against second press base 12b. Additional tightening of nut 22 places tie bar 18 under tension, which, when combined with other tensioned tie bars 18 positioned between the remaining pairs of adjacent press bases 12, provides a measure of stability to cubic press 10. Pre-loading or pre-tensioning tie bars 18 in this manner (i.e., tensioning the tie bars 18 prior to operation of the press 10) can increase the fatigue life of tie bars 18.
Problems with the above configuration arise in that the large diameter of tie bar 18 makes tightening nut 22 about tie bar 18 extremely difficult. The torque required to tighten a nut about a bolt is may be approximated by the equation T=(c)D3, where T is the torque required, c is a constant, and D is the diameter of the bolt. Accordingly, as bolt diameter increases, an exponential increase in torque is required to tighten the nut about the bolt. Because tie bars 18 used in cubic press 10 often have relatively large diameters (including tie bars with diameters of 9 inches or larger), the amount of torque required to tighten nut 22 on an associated tie bar 18 is extremely high and, in some circumstances, impossible or impractical to produce given resource limitations. When the amount of torque required to further tighten nut 22 about tie bar 18 becomes too large, the amount of tension (or pre-tension) supplied to tie bar 18 is effectively capped or limited, despite the fact that additional tensioning might increase the fatigue life of tie bars 18.
Additionally, while precise pre-tensioning of tie bars 18 is desirable, the above-described configuration often requires crude tightening techniques with little control over the amount of torque provided to nut 22. For example, a common procedure for tightening nuts about large bolts is to secure a long-armed wrench to the nut and then hit a sledge hammer against the arm of the wrench to twist the nut about the bolt. Great variations in the amount of torque supplied by each swing of the sledgehammer against the arm of the wrench results in little or no control over pre-tensioning accuracy.
Further, it has been found that threads at an end of tie bars 18 are often the location of low cycle fatigue failure. Because the above-described configuration relies upon nut the 22 tightened onto threaded tie bar 18 to pre-tension tie bar 18, tie bars 18 are susceptible to low cycle fatigue failure.
Thus, it would be advantageous to provide an improved high pressure press assembly and an improved method of assembling a high pressure press.